1. Introduction. ABSENCE OF FIELD COOLING EFFECT ON THE HYSTERESIS LOOP IN AMORPHOUS F~3Zr7

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1 ABSENCE OF FIELD COOLING EFFECT ON THE HYSTERESIS LOOP IN AMORPHOUS F~3Zr7 L.F. KISS and D. KAPTAS Research Institute/or Solid State Physics H-1525 Budapest P.O.Box 49, Hungary N. HEGMAN Institute ofnuclear Research H-4001 Debrecen, P. O.Box 51, Hungary ABSTRACT. Spin-glass-like amorphous Fe93Zr7 was cooled in zero field and in a field of 2 T to a temperature of about 6 K and the corresponding hysteresis loops were measured at this temperature up to a field of 1 T. In spin glasses with partial antiferromagnetic spin order, e.g. crystalline CuMn, a shift of the hysteresis loop is expected due to exchange anisotropy. No shift of the loop along the field axis was observed indicating the absence of a unidirectional anisotropy. The absence of the field cooling effect in F~3Zr7 points to the absence of any significant anti ferromagnetic interactions, in agreement with previous magnetic and Mossbauer data on the same alloy. 1. Introduction The amorphous Fe1oo-xZr x (7 ~x~ 12) alloy series is one of the most intensively studied amorphous TM-TM (TM = transition metal) alloy systems because of its peculiar magnetic properties below room temperature (RT). In these alloys, a continuous change can be observed from a spin-glass-like behaviour for x = 7, characterized by a single transition temperature at T g = 105 K, through a reentrantspin-glass regime for 8::;;x~ 10 to a ferromagnetic behaviour for x = 12 [1,2,3]. Several attempts have been made in the literature to explain this peculiar magnetic behaviour of the amorphous Fe-Zr system at low temperatures. The existence of an inhomogeneous magnetic structure is often assumed in these alloys: according to these models, antiferro magnetic (AF) [4] or ferromagnetic (FM) [5] clusters embedded in the ferromagnetic matrix are responsible for the anomalies. Other authors [6] use a homogeneous model: exchange frustration caused by the simultaneous presence of FM and AF exchange interactions leads to a complicated non-collinear spin arrangement, to the freezing of the x,y components of the spins. C.C. Hadjipanayis (ed), Magnetic Hysteresis in Novel Magnetic Materials, ~ 1997Kluwer Academic Publishers. Printed in the Netherlands. 755

2 756 A common feature of practically all of the models is the assumption of AF exchange interactions in the alloys. It is based on the known strong decrease of the direct exchange integral between Fe-Fe atoms with decreasing Fe-Fe separation, changing sign at about 2.55 A from FM to AF exchange [7]. Since the Fe-Fe separation is widely distributed around a value close to this critical separation because of the amorphous structure of the Fe-Zr alloys, exchange interactions ranging from negative (AF) to positive (FM) values are assumed in these alloys. These considerations seem to be plausible because the presence of mixed (FM and AF) interactions.is known to play an important role in some of the canonical (crystalline) spin glasses, e.g. CuMn, AgMn, CoMn, etc. Ferromagnetic clusters are also known to exist in some of these classical spin glasses, e.g. CoMn, AgMn, AuFe, etc. Both ingredients of the magnetic structure are present in the most typical spin glass, CuMn: (i) magnetic clusters with strongly FM spin correlation, giving rise to very large dipole moments and (ii) a matrix with AF short range (and in some cases even long range) spin order [8]. These are thought to lead to the full range of properties typical of spin glasses: displacement of the hysteresis loop and unidirectional remanence after field cooling to temperatures of the order of T g/30, thermomagnetic history effects and time-dependence around T g/3 and a more-or-iess sharp ac low field susceptibility maximum at T g CoMn alloys, representing one extreme case, were shown to have a massive AF short range spin order, but with only very small and few magnetic (FM) clusters [9]. Here the unidirectional remanence as well as the thermomagnetic history effect and the susceptibility maximum all tend to vanish, although the displacement after field cooling can be very large. The other extreme is represented by the AuFe alloys which have a large fraction of their atomic moments arranged in magnetic clusters and they have very little AF matrix, if any [9,10]. Here the susceptibility maximum is very high indeed and the thermomagnetic history effects are very prominent, but the displacement and the unidirectional remanence are practically absent or occur only at extremely low temperatures. There are, however, strong indications against the existence of AF exchange interactions in the amorphous Fe-Zr alloys [11,12]. The above considerations concerning the canonical spin glasses suggest a simple method to confirm or to reject the presence of AF exchange interactions in Fe-Zr. If these were present in the alloys, the displacement of the hysteresis loop after field cooling to temperatures of the order of T g /30 should be observed. There are a few such measurements published in the literature for Fe92Zrg [1] and Fe90ZrlO [13]. Both show the shift of the hysteresis loop after field cooling to 4.2 K, the former by 28 % with respect to the coercive field, H c ' Since T g = 60 K and 15 K for Fe92Zrg and Fe90ZrlO' respectively, the measuring temperature used (4.2 K) by these authors is much higher than T g /30. The development of field-cooled magnetic anisotropy in F~2Zrg was also reported [14,15]. More pronounced field cooling effects are expected for amorphous Fe93Zr7' having a T g of 105 K, at measuring temperatures close to T g /30. To check this implication, in this contribution the hysteresis loops were measured for amorphous Fe93Zr7 after cooling in zero field and in a field of 2 T to about 6 K.

3 Experimental details The amorphous Fe93Zr7 alloy, prepared in ribbon form of the geometry 1 mm x 12 JJ.m by melt-spinning in Ar atmosphere, was checked to be amorphous by x-ray and RT Mossbauer measurements. 10 pieces, each 5-6 mm long, weighing about 3 mg, were used for the measurement of the hysteresis loop. The hysteresis loops were measured between the fields of ± 1 T in a bipolar 5 T superconducting magnet with a 2-5 s waiting time between polarity changes. The magnetization was obtained by an extraction magnetometer, having two pick-up coils between which the sample was moved with a frequency of 2 Hz by a motor. The magnetization was calibrated by a solenoid of a geometry close to that of the sample. 3. Results The coercive field, He' for the amorphous F~3Zr7 is known to increase very rapidly with decreasing temperature below about T g [4]. At 4.2 K its coercive field amounts to about 1000 Oe, Figure 1 shows the hysteresis loops for Fe93Zr7 after cooling in zero field from RT to 5.5 K (circles) and in a field of 2 T from 135 K (i.e. from above T g ) to 6.1 K (triangles). The drop in the magnetization at zero field is due the time variation of the magnetization during the waiting time between polarity changes. No significant displacement along the field axis can be observed, showing that no unidirectional anisotropy (attributable to an exchange anisotropy) develops after field cooling in the alloy. This suggests that F~3Zr7 has very little AF matrix, if any. Since its susceptibility maximum is high [3] and thermomagnetic effects are also known for the amorphous Fe-Zr alloys [1], this system seems to have a similar magnetic structure like the crystalline AuFe alloys. 4. Discussion Besides the absence of unidirectional anisotropy after field cooling in Fe93Zr7' Mossbauer measurements performed in magnetic fields up to 7 T applied parallel to the gamma-beam strongly question the existence of AF exchange interactions in the amorphous Fe-Zr system [12]. The six lines observed in a magnetically split spectrum have intensities 3:R:l:I:R:3, where R = 4 sin 2 e /(1 + cos 2 e ) and e is the angle between the magnetic moment and the direction of the gamma-beam. R was measured to be zero above 6 T at 4.2 K for all the amorphous Fe-Zr alloys, indicating that each alloy has a collinear spin structure above 6 T. If there were AF exchange interactions present in the alloys, a field of about several hundred Tesla would be required to overcome the AF anisotropy. A second indication against the presence of AF interactions in these alloys comes from the fact that similar values were obtained for the high-field susceptibility

4 758 from both Mossbauer and bulk magnetization measurements for each composition [12]. Otherwise, the latter should be larger than the former because of the presence of reversed spins with respect to the external field a ZFC T=5.5 K from RT) ~~~~~ Fe T=6.1 K from 135 K at 2 T) Fe~31r7. I r-' I I I L L L L L --~--~--~--~ --l--~--~--~-- I I I I I I I ~ I I I ~ CJ I I I -_L_-L_-l-~-_L_gl--L L--L_- --~--~--~-~-- -~--~--~--~-- I I I ~ I I I I I I I I L L L L L L - 15 O~~ ~ e H (1 0 x koe) Figure 1. Hysteresis loopsfor Fe9jZr7after cooling in zero field (Z,FC) to 5.5 K (circles) and in afield of 2 T (FC) to 6.1 K (triangles). 5. Conclusion No evidence was found in amorphous spin-glass-like Fe93Zr7 with a T g of 105 K for the displacement of the hysteresis loop after field cooling to about 6 K, indicating the absence of AF exchange interactions in the alloy. This result is supported by highfield Mdssbauer measurements. ACKNOWLEDGMENTS The authors are indebted to L. Bujdos6 for preparing the sample. The financial support of the Hungarian Research Fund through OTKA grants Nos. F and T is highly acknowledged.

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